1. Field of the Invention
The present invention relates to a current mirror circuit, and more particularly to a current mirror circuit that automatically switches a suitable amplifying rate according to a present input bias current.
2. Description of Related Art
In general, a current mirror is composed of multiple transistor elements. One type of current mirror uses the MOSFETs as the transistor elements. Since the material characteristics of the MOSFETs, supplying different bias currents to the current mirror affects accuracy of the output current of the current mirror.
The MOSFETs applied in the current mirror are mostly operated in the saturation region. The simple relationship between the source current Ids and the gate voltage Vgs of the MOSFET is presented Ids=[μCOX(W/L)(Vgs−Vth)2/2] when the MOSFET is operated in the saturation region. The parameters μ, COX, W, L and Vth of each MOSFET are determined in fabricating procedure, so the parameters of the MOSFETs of the current mirror are different. Product of μCOX(W/L) for each MOSFET of the current mirror is not changed wildly, but value of (Vgs−Vth) is changed when different bias currents are input to the current mirror. That is, the value of (Vgs−Vth) is decreased when a small input bias current is input to the current mirror. Since the material characteristics of the MOSFET, the parameter Vth is not stable when the MOSFET is activated for a long time. Therefore, the unstable Vth directly affects the source current Ids. That is, the lower bias current signal causes the higher error of the output current of the current mirror.
With reference to FIG. 15, a real current mirror circuit device is a two-stage configuration including two current mirrors connected in series. The first stage current mirror is composed of two MOSFETs (M1, M2) and has an amplifying rate being 10:1, an input and an output. The second stage current mirror is composed of the three MOSFETs (M3, M4, M5) and has an amplifying rate being 1:10, an input connected to the output of the first stage current mirror and two outputs.
Assuming an input current (IIN) supplied to the input of the first stage current mirror is 100 μA, the bias current (IB) generated in the output of the first stage current mirror is 10 μA according to the amplifying rate (10:1) of the first stage current mirror. The bias current (IB) is supplied to the input of the second stage current mirror and then the second stage current mirror generates two output currents (IOUT1, IOUT2) (100 μA) in the two outputs according to the amplifying rate (1:10). Furthermore, if the input current (IIN) supplying to the input of the first stage current mirror is 10 μA, the bias current (IB) generated in the output of the first stage current mirror is 1 μA. The second stage current mirror then generates two output currents (IOUT1, IOUT2) (10 μA) in the two outputs. Therefore, the error of the output currents (IOUT1, IOUT2) increase when a smaller bias current is input to the input of the second stage current mirror.
Regarding to current applications with high accuracy requirement, such as driving circuit of the LED or OLED etc. product, the error of the output current is hard to ignore and is more and more important. Since the driving circuit of the OLED product has to generate multiple small driving currents, the error between the input current of the OLED product and driving currents and skews among driving currents are obviously higher and worse than that of the input current and the driving currents generated by other driving circuit of other product operated in large signal mode. Furthermore, a range of the output current is limited because a maximum bias current is limited according to limitation of the MOSFET standard. If the bias current input to the MOSFET is much lower than the maximum bias current, the error of the output current extremely increases and a large error of the output current limits the improvement for driving circuit having high accuracy requirement.
To overcome the shortcomings, the present invention provides a current mirror circuit that automatically switches a suitable amplifying rate to mitigate or obviate the aforementioned problems.